Determination of the optimal coverage for heavy-duty-axle gears in shot peening

Abstract

Pitting and wear often appear on heavy-duty-axle gears due to their harsh working conditions, such as high torques, high loads, and poor lubrication. Shot peening is a popular surface strengthening method for gears. In order to ensure complete coverage during shot peening, 100~200% coverage is usually prescribed for most gears. However, it is difficult to effectively improve the contact fatigue and wear resistance of heavy-duty-axle gears. Generally, increasing shot peening coverage can heighten the compressive residual stress for prolonging the service lifetime of gears, whereas high coverage levels may cause the deterioration of surface roughness, thus increasing the noise and vibration of gears. To address this issue, this paper deals with the determination of optimal coverage for heavy-duty-axle gears by experimental tests. The influence of shot peening coverage on the surface integrity of gears is analyzed in terms of residual stress, microhardness, surface morphology, and microstructure. The results show that the maximum compressive residual stress increases first and then keeps stable with the increase of coverage, and the maximum value is − 1172.10 MPa. The microhardness peak increases obviously in the beginning and then slowly rises with the increase of coverage, and the maximum value is 747.5 HV1.0. The surface roughness (Ra) decreases initially and then enhances with the increase of coverage, and the minimum value is 0.99 μm under the coverage of 1000%. The crystallite size can be refined from 36.88 to 28.79 nm by shot peening. The dislocation density increases with the increase of coverage, and the maximum value is 3.70 × 1016 m−2. Numerous damages (microscalings, spallings) occur on the treated gear tooth flank affecting the residual stress distribution and roughness under high coverage levels. Meanwhile, the wear behavior of gear steels is investigated, and the wear test results show that shot peening with the coverage of 1000% can lead to a better wear resistance. Taking into consideration of surface integrity and wear test results, the coverage of 1000% is the optimal coverage for heavy-duty-axle gears in shot peening.